VERITAS Volume Manager 4.1 Administrator's Guide

Administering Cluster Functionality
Overview of Cluster Volume Management
Chapter 10 355
Each physical disk is marked with a unique disk ID. When cluster
functionality for VxVM starts on the master, it imports all shared disk
groups (except for any that have the noautoimport attribute set). When
a slave tries to join a cluster, the master sends it a list of the disk IDs
that it has imported, and the slave checks to see if it can access them all.
If the slave cannot access one of the listed disks, it abandons its attempt
to join the cluster. If it can access all of the listed disks, it imports the
same shared disk groups as the master and joins the cluster. When a
node leaves the cluster, it deports all its imported shared disk groups, but
they remain imported on the surviving nodes.
Reconfiguring a shared disk group is performed with the cooperation of
all nodes. Configuration changes to the disk group happen
simultaneously on all nodes and the changes are identical. Such changes
are atomic in nature, which means that they either occur simultaneously
on all nodes or not at all.
Whether all members of the cluster have simultaneous read and write
access to a cluster-shareable disk group depends on its activation mode
setting as discussed in “Activation Modes of Shared Disk Groups” on
page 355 The data contained in a cluster-shareable disk group is
available as long as at least one node is active in the cluster. The failure
of a cluster node does not affect access by the remaining active nodes.
Regardless of which node accesses a cluster-shareable disk group, the
configuration of the disk group looks the same.
NOTE Applications running on each node can access the data on the VM disks
simultaneously. VxVM does not protect against simultaneous writes to
shared volumes by more than one node. It is assumed that applications
control consistency (by using a distributed lock manager, for example).
Activation Modes of Shared Disk Groups
A shared disk group must be activated on a node in order for the volumes
in the disk group to become accessible for application I/O from that node.
The ability of applications to read from or to write to volumes is dictated
by the activation mode of a shared disk group. Valid activation modes for
a shared disk group are exclusive-write, read-only, shared-read,
shared-write, and off (inactive). These activation modes are described
in detail in the table Table 10-1, “Activation Modes for Shared Disk
Groups,” on page 356.